Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro

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On 3/16/2021 8:49 AM, Dave Hansen wrote:
On 3/16/21 8:13 AM, Yu-cheng Yu wrote:
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -392,3 +392,21 @@ For 32-bit we have the following conventions - kernel is built with
  .endm
#endif /* CONFIG_SMP */
+/*
+ * ENDBR is an instruction for the Indirect Branch Tracking (IBT) component
+ * of CET.  IBT prevents attacks by ensuring that (most) indirect branches
+ * function calls may only land at ENDBR instructions.  Branches that don't
+ * follow the rules will result in control flow (#CF) exceptions.
+ * ENDBR is a noop when IBT is unsupported or disabled.  Most ENDBR
+ * instructions are inserted automatically by the compiler, but branch
+ * targets written in assembly must have ENDBR added manually.
+ */
+.macro ENDBR
+#ifdef CONFIG_X86_CET
+#ifdef __i386__
+	endbr32
+#else
+	endbr64
+#endif
+#endif
+.endm

Is "#ifdef __i386__" the right thing to use here?  I guess ENDBR only
ends up getting used in the VDSO, but there's a lot of
non-userspace-exposed stuff in calling.h.  It seems a bit weird to have
the normally userspace-only __i386__ in there.

I don't see any existing direct use of __i386__ in arch/x86/entry/vdso.


Good point. My thought was, __i386__ comes from the compiler having the -m32 command-line option, and it is not dependent on anything else.

Alternatively, there is another compiler-defined macro _CET_ENDBR that can be used. We can put the following in calling.h:

#ifdef __CET__
#include <cet.h>
#else
#define _CET_ENDBR
#endif

and then use _CET_ENDBR in other files.  How is that?

In the future, in case we have kernel-mode IBT, ENDBR macros are also needed for other assembly files.

Thanks,
Yu-cheng



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